Martensitic steel

ABSTRACT

Martensitic, age hardenable stainless steel substantially free of delta ferrite, containing traces to about 0.07 percent carbon, traces to about 0.35 percent silicon, traces to about 0.6 percent manganese, from about 14 percent to about 17.5 percent chromium, from about 4.75 percent to about 7 percent nickel, from about 0.3 percent to about 1.3 percent titanium, and the balance essentially iron with residuals and residual impurities and in which the steel is characterized by exhibiting a ratio of notch tensile strength to smooth tensile strength of at least 1.

United States Patent Lula et al. [4 1 Apr. 25, 1972 [541 MARTENSITIC STEEL 1,538,337 5/1925 Kuehn ..15/12s T [72] Inventors: Remus A. Lula, New Kensington; George gg Sarver, both of Charles M.

Hammond, Alliance, i OTHER PUBLICATIONS [73] Assignee: Allegheny Ludlum Steel Corporation, Firth Vickers F. V. 520 Alloy Data Sheet Code No. SS 84 Brackenridge, Pa. Alloy Digest Published by Engineering Alloy Digest Inc. [22] Filed: Oct. 8 1968 Montcla1r N.J., Feb. 1959 [21] Appl. No.: 765,766 Primary Examiner-Hyland Bizot Attorney-Richard A. Speer, Vincent G. Gioia and James A. Related U.S. Application Data Berneburg [63] Continuation-impart of Ser. No. 316,01 1, Oct. 14,

1963, abandoned. [57] ABSTRAT Martensitic, age hardenable stainless steel substantially free of [52] U.S. Cl ..75/128 T delta f rri n ini g traces to about p n n. 51] ]m (1, l C22 39/20 traces to about 0.35 percent silicon, traces to about 0.6 per- [58] Field of Search ..75/l28, 128.8; 14/37, 142 cent manganese, from about 14 percent to about Percent chromium, from about 4.75 percent to about 7 percent nickel, [56] References Cited from about 0.3 percent to about 1.3 percent titanium, and the balance essentially iron with residuals and residual impurities UNITED STATES PATENTS and in which the steel is characterized by exhibiting a ratio of notch tensile strength to smooth tensile strength of at least 1. 2,395,608 2/1946 Aborn ..l48/142 3,251,683 5/1966 Hammond ..75/128.8 3 Claims, 1 Drawing Figure Annealed i Annealed a Aged l 1 t /0 q Aged l8 8 i o s Hot Rolled l l l l DRILL SPEED (.S'FM/ Patented April 25, 1 9-72 Annealed Annealed Hot Rolled DRILL SPEED (.SFM)

A d0 8L N RUGO L W NM u N m n m N M r E A 5 RM W U 0.0". 6 .6 e y I MARTENSITIC STEEL CROSS REFERENCES TO RELATED APPLICATIONS This is a continuation-in-part of Ser. No. 316,01 lfile'd Oct. 14, 1963, now abandoned.

This invention relates to a martensitic, age hardenable stainless steel, and in particular to a martensitic, .agehardenable steel which is suited for fabrication both by forming and joinmg.

-Recently a new series of steels have come into prominence for structural applications. These steels are characterized by the fact that they are predominantly martensitic steels and combine a phase transformation phenomenon with an age hardening reaction in order to develop their ultimate mechanical properties. These steels, insome' instances, possess little corrosion resistance; consequently',theyare limited from an application standpoint to a structuralrapplication wherein corrosion resistance is of noimportance. In 'addition, there are twostainless varieties whichare commercially available and which are .age hardening-steels 'having a multiphase microstructure. This microstructure' includes retained austenite, delta ferrite and martensite. As a result ofthis'multiphase microstructure some of the mechanicalproperties'are impaired. These steels are also characterized by exhibiting poor ductility and are notch sensitive in the transverse direction.

The steel of the present invention is'classified as a Maraging stainless steel. In the annealed condition this steel is substantially completely martensiticf Because of the single phase microstructure, low initial carbon and nitrogen contents and the addition of titanium, this steel is characterized byhaving excellent fabrication characteristics such as drawability,

" machinability and weldability. The steel is strengthenedby a simple, low temperature aging treatment.

An object of this invention is to provide a martensitic, age hardenable stainless steel having an optimum combination of corrosion resistance and strength.

Anotherobject of this invention is to provide amartensitic,

age hardenable stainless steel which possesses ahigh degree of toughness.

Other objects of this invention will become apparent to'one skilled in the'art when read in conjunction with the following description and drawing, the single FIGUREofwhich illus- ='trates the machinability characteristics ofthe'steel: of'the present invention.

The steel-of thepresent invention has a composition as set 1 forth withinthe following TABLE I:

TABLE I Chemical Composition byWt.)

Element General Range Preferred 'Range Traces 0.07 Traces 0.05

Si Traces 0.35 Traces 0.20

Mn Traces 0.6 Traces 0.3

Ti or Cb 0.34.3 0.4-1.0

Fe Bal. Bal.

his to be noted in Table I that both a general range and a preferred range are set forth for each of the alloying com- -ponents present within the steel. Carbon, which is present withinrthe steel of the present invention, contributes to the formation of austenite, which phase must be formed during the high temperature annealing heat treatment in order to obtain a-rnartensitic structure upon cooling the steel to room 1 bon martensite is formed upon cooling the steel to room temperature. The low carbon martensite will have sufficient softness andductility to enable'the ultimate user to fabricate the steel of the present invention by any of the presently recognized and used means. Where the carbon content is increased to above about 0.07 percent, it has been found that excess carbon removes titanium by forming titanium carbonitrides within'the steel, thus detracting from the quality thereof. Moreover, higher carbon contents tie up sufficient titanium so that it is removed from its useful function as forming age hardening precipitates, thereby adversely affecting the strength of the steel. Accordingly, the carbon content is preferred to be maintained as low as possible, as set forth in Table I.

The steel of the present invention contains silicon within the range between traces to about 0.35 percent. Silicon has been found to be a potent strengthener when taken into solid solution, and in this respect the silicon content of the present invention must be critically controlled for its effect upon the fabrication properties, as well as the strength of the steel itself.

The silicon content is preferred to be maintained at less than about 0.20 percent in order to obtain optimum properties. It has been found that these lower silicon contents are desirable in order to obtain sufficient fabrication properties in the an- -nealed condition and notch ductility in the aged condition within the steel'of the present invention. The silicon content present within the solid solution of the steel of the'present invention, while greatly contributing to the strength of the steel in the aged condition, nonetheless adversely affects the toughness "and formability when the silicon content is increased to amounts beyond that stated in Table I.

Manganese alsoappears to be'effective in this respect, and together with the siliconmust be critically controlled in order to obtain the optimum toughness characteristics. In this respect, it has been found that each of the silicon'and manganese must be-limited. The silicon must be maintained at less than 0.35 percent and the manganese may be presentup to 1 about 0.6 percent in order to-achieve a ratio of notch tensile =strength(NS) to smooth tensile strength (UTS) of at least 1. Where the silicon content is less than 0.35 percent, the steel of the present invention exhibits aratio of the notched tensile strength to the-ultimate tensile strength of about 1 or greater,

irrespective of the fact that the steel is heat treated to substantiallythe same strength level'or is given a standard aging'heat i must bemaintained within the ranges set forth in Table I.

treatment. Consequently,both the'manganese and the silicon The steel of the present invention contains chromium .within the rangeof between about 14 and 17.5 percent, and

"preferably within the range between 14.5 and 15.5 percent, in order to obtain an optimum corrosion resistance consistent with mechanical-properties. While chromium is a strong ferrite former, it is balanced with the other alloying components in order to obtain a completely martensitic structure when the steel is cooled to room temperature or below after annealing.

The chromium content, as thus limited within the ranges set forth hereinbefore in Table 1, provides the optimum combination of corrosionresistance, rnicrostructureand mechanical properties.

preferably between 5.25 and 6.5 percent, and unites with titanium to form an age hardening precipitate. Nickel is a strong austenite forming element, and thus balances the chromium content, thereby preventing the formation of delta ferrite upon heating the steel to the annealing heat treatment temperature. The nickel content, however, must be maintained at less than about 7 percent in order that the austenite which is formed will have Ms and Mf temperatures which are above room temperature for the steel to transform to martensite upon cooling to normal room temperature. Where the nickel content is increased to beyond about 7 percent, part of the austenitic phase may be retained at room temperature, thereby adversely affecting the strength of the steel. On the other hand, at least 4.75 percent nickel is necessary to prevent the formation of delta ferrite during the annealing heat treatment. Optimum results are obtained when the nickel content is maintained within the preferred range set forth hereinbefore in Table l, in that the structure of the steel is balanced so as to prevent the formation of delta ferrite or of retained austenite, yet without unduly depressing the Mf or Ms temperature.

Titanium is preferred to be maintained within the range between 0.30 and 1.3 percent and performs the dual function of tying up the carbon and combining, inter alia, with nickel to form an age hardening precipitate, which is the main strengthening mechanism of the steel of the present invention. At least 0.30 percent titanium is required, whereas titanium contents in excess of about 1.30 percent do not appear to provide any outstanding increase in the age hardening characteristics of the steel, and may adversely affect the structure and mechanical properties. While optimum results are obtained where the titanium content is maintained within the preferred range set forth in Table I, the titanium content should be increased where the carbon content is increased, each within the limits set forth hereinbefore in Table I. It is to be noted that columbium may be substituted for a part or all of the titanium, and functions in the same manner, that is, it performs the dual function of tying up the carbon and nitrogen and forming an age hardening precipitate. Consequently, the range set forth in Table I for titanium is also applicable to columbium.

The steel of the present invention may also optionally contain some molybdenum, e.g. residual to about 2 percent. Molybdenum performs a dual function in contributing to the solid solution strengthening of the steel, and in affording an extra measure of corrosion resistance, especially where the steel of the present invention is used in atmospheres containing a compound involving a halogen ion. As thus utilized for this particular application, molybdenum may be present in amounts of up to 2 percent without changing the characteristics of the steel of the present invention. Where, however, the steel is not intended for an application involving the halogen ion or for extreme stress corrosion resistance, the molybdenum may just as readily be omitted from the steel without adversely affecting the basic corrosion resistance or the basic strength of the steel.

The balance of the steel comprises essentially iron with the normal incidental impurities usually found in the production ofstainless steels.

The steel of the present invention may be made in any of the well-known manners, for example by carbon electrode electric arc melting. In this respect, scrap and/or hot metal may be charged into a conventional furnace and melted to the desired analysis, from which the molten steel is tapped and teemed into ingots. After proper soaking, the ingots may be hot rolled either to bar products or flat rolled products, following which they may be subjected to annealing, descaling and cold reduction. The steel of this invention, having been produced in a commercial electric arc furnace in amounts ranging between 10 and 70 tons, has been cold rolled in strip form to beyond 75 percent reduction in the cross sectional area from the annealed and pickled hot rolled thickness without any difficulty having been encountered. The same steel, when formed into seamless tubing, has been cold drawn to effect an 80 percent reduction in wall thickness with similar success, whereas the steel in the form of wire has been cold drawn to effect a 99 percent reduction in the cross sectional area without subjecting the steel to an intermediate anneal.

The steel in either bar or flat rolled product fonn is preferably annealed by heating to a temperature within the range between about 1,400 F. and about 1,600" F although temperatures of up to 2,000 F. can be used. The steel is cooled to room temperature following such high temperature annealing heat treatment. As annealed, the steel is characterized by a substantially completely martensitic microstructure having a low carbon content and excellent transverse ductility. In this form the steel may be descaled and fabricated to the finished shape, following which the application of a low temperature aging heat treatment develops the optimum mechanical properties within the steel in its fabricated form. The preferred aging heat treatment consists of subjecting the steel to a temperature within the range between about 800 and 1,l50 F. for a time period ranging between about 5 minutes to several hours, depending upon the strength desired and the cross sectional area of the finished product. After aging, it is preferred to air cool the aged steel in the normal manner. It is to be noted that with the use of the low temperature aging heat treatment, no distortion or heavy discoloration is apparent as a result of the heat treatment. While the steel is easily weldable and exhibits excellent ductility in the aswelded condition, the same aging treatment is effective for age hardening the weldment. The steel of the present invention also exhibits outstanding machinability in the age hardened condition. In this respect, the producer can supply the bars in the fully aged condition from which a part may be machined with great ease. Since no further heat treatment is involved, subsequent descaling is not required and the dimensional tolerance can be precisely controlled, irrespective of the fact that the aging heat treatment does not produce distortion in a fully fabricated part.

In order to more clearly demonstrate the advantages of the steel of the present invention, reference is directed to the following Table II which illustrates typical properties of the steel in sheet form. The mechanical properties reported in Table 11 are those resulting from tests performed on heat GL-58 which had a nominal composition of 0.02 percent carbon, 0.10 percent manganese, 0.13 percent silicon, 15.05 percent chromium, 5.7 percent nickel, 0.75 percent titanium and the balance essentially iron with incidental impurities.

As noted from Table II, the steel in the annealed condition has a yield strength of about 99.5 Kpsi, and an elongation in a 2- inch gauge length of about 8 percent. The mechanical properties of the steel following aging clearly illustrate that the material has an outstanding response to age hardening. The preferred aging temperature is efiective in most instances for increasing the hardness by about 15 R The corresponding tensile tests clearly reveal that the age hardening heat treatment is effective for producing an outstanding increase in the 0.2 percent yield strength and the ultimate tensile strength without adversely affecting the ductility, as measured by the percentage elongation, to any great degree. These mechanical properties as set forth in Table 11 make the steel particularly attractive for structural applications where good corrosion resistance, commensurate with mechanical strength, is desired.

In order to more clearly demonstrate the effect of the alloying elements on the strength of the material, reference is directed to the following Table III which lists a series of compositions having a predetermined variation of silicon and manganese with the balance of the alloy components remaining substantially constant. Heat GM-S, shown therein, is a comexample, the test results shown for heat GL-64. Also, by maintaining the silicon content within the optimum range but increasing the manganese content outside its individual range, it is clear that the ratio of notch tensile to smooth tensile position presently commercially available, which contains a 5 strength is less than 1. multi-phase microstructure which includes martensite, delta Heat GM-S, which is a commercially available alloy, clearly ferrite and retained austenite. illustrates an inferior ratio of notch tensile strength to smooth tensile strength, which is believed to be related to the fact that TABLE 111 this steeljcontains a multi-phase structure consisting of mar- (3 M11 Si or Ni Ti tensite, delta ferrite and retained austenite, and higher .023 10 13 mos 5.70 .74 amounts of silicon and manganese. From the foregoing it is .015 15,00 572 clear that a close control of the composition is necessary 1n :81; 832 :33 1g: 3g g: g; :32 order to ensure outstanding toughness at high strength levels. 1 1:1 g. 75 l 5 In order to substantiate the conclusions derived from the UM 1, I055 I 50 16:40 :52 test results set forth 1n Table IV and to further demonstrate the criticality of the composition, additional heats were made The steels from the heats having the chemical composition gg z -i-gi gg heats havmg the composmons set set forth in Table III were rolled to sheet material and heat treated by subjecting the same to a temperature of about TABLE V l,500 F. for a time period of 10 minutes and thereafter cool- 0 Mn 51 (g x 1 ing. Since silicon is a potent solid solution strengthener and .045 .14 .17 15.15 5.70 .10 contnbutes to age hardenmg, and s1nce it was des1red to evalu- 14 15. 20 5. 73 .75 ate toughness at equivalent strength levels, various aging treat- -g% 38 gments were used. The measure of toughness employed is the .012 .47 .080 15: 5:00 :64 .014 .26 .37 15.17 5.00 .80

ratio of the notched tensile strength to the smooth tensile strength as recommended by the ASTM Committee on Fracture Toughness (ASTM Bulletin No. 243, January, 1960, and No. 244, February, 1960). Table IV, set forth hereinafter, records the results of these tests, it being noted that each of the steels was tested in the transverse direction at room temperature after aging at the temperatures and for the time periods set forth in Table IV:

TABLE IV Smooth tenslles Hard- Percent Heat ness, Testing N.S., 2% Y.S., UTS, elongation N .S./ Number Aging treatment Re temp. K p.s.1. K p.s.i. K p.s.i. in 2" UTS GL-58A 900 F., 8 hrs. AC 41 R.T. 202. 7 172.0 188.5 5.0 1. 10 GL-GOA 800 F., 3 hrs. AC 39 R.T. 182.5 162.3 168.8 5.0 1.08 GL-60B 800.F., 3 hrs. A 39 R.'I. 171. 5 164. 6 173. 7 6.0 0.98 GL.64 800 F., 2 hrs. AC 39. 5 R.T. 173. 3 163. 5 174. 8 5. 2 0.99 GM-47 900 F., 8 hrs. AC 41. 5 R.T. 180.9 180. 6 195.2 4.0 0.93 GM-50 900 F., 8 hrs. AC 41 R.T. 174. 6 187.0 4. 0 0.93 G.\I5 800 F., 1 hr. AC 41 R.T. 159.3 170.6 184. 4 9.5 0.86

Heat GL-S 8A set forth in Table IV illustrates fairly typical properties for the steel of the present invention as compared to the data set forth in Table II. It is noted that both man- Attention is directed to the fact that in Table VI, as set forth hereinafter, a constant aging temperature has been utilized as contrasted to the data set forth in Table IV where .an

ganese and silicon are within the specified ranges set forth 50 equivalent strength was desired, necessitating the variation in hereinbefore in Table 1. Test results for heat GL-58A clearly the aging temperature.

TABLE VI Smooth tensiles Hard- ---Elongatlon Heat ness, Testing N.S Y.S., UTS, percent N.S./ Number Aging treatment (1 temp. K p.s 1 K p.s.i. K p.s.i. in 2 UPS GT-100 900 F., 8 hrs. AC 39. 5 R.T. 270.0 165. 7 167. 0 14. 0 1. 62 GV38 900 F., 8 hrs. AC 45. 0 R.T. 272.0 195. 7 199. 9 17. 0 1. 36 GV-78... F., 8 hrs AC 47. 0 R.T 191. 8 204. 3 208. 4 15.0 92 GV-SO... 900 F., 8 hrs AC 47. 0 R.T 208.0 215.0 218.8 13.0 .95 GV1 8 hrs. AC 39 R.T 274.4 170.2 173.0 20.0 1.58 GV-96 900 F., 8 hrs. AC 45 R.T 199. 8 205. 6 210. 4 14. 5 .05

demonstrate a ratio of notch tensile strength to smooth tensile 5 From the test results recorded in Table V1 for heats GT-l00 strength of 1.10. By increasing the silicon content to the upper limit, that is, 0.35 percent max., and maintaining the manganese low, and by under-aging the steel in order to obtain the equivalent strength level, it is clear that little change is noted in the ratio of the notch tensile strength to the smooth tensile strength. Where, however, the silicon content is increased to about 0.58 percent, which is in excess of the upper limit, it is clear that heat GL-B attains a ratio of notch tensile strength to smooth tensile strength of less than 1. Further increases in and GV-38 it is seen that increasing the silicon content from 0.17 percent to 0.34 percent is effective for producing an increase in both the ultimate tensile strength and the 0.2 percent yield strength. When materials from these same heats having a notch machined into the reduced section of the test specimen were evaluated, it is seen that the ratio of the notched tensile strength to the smooth tensile strength was in excess of one. However, where the silicon content was increased to 0157 percent and'0.86 percent with substantially the samemanganese the silicon content do not apparently change this ratio, as for content, for example in heats GV-78 and GV-80, it is noted that the ratio of notch tensile strength to smooth tensile strength decreased to below one. Heat GV-l, which has a low silicon content and a high manganese content, clearly illustrates the criticality of the composition of the steel of the 8 From the test results set forth in Table VIII it is clear that the steel of the present invention possesses excellent corrosion resistance in the media set forth therein. This is indicative of the overall corrosion resistance exhibited by the steel of the present invention. Where the silicon content exceeds about present invention. Further corrosion tests were also employed 0.35 percent, as for example heat GV-96 which contains 0.37 in which the steel of the present invention was subjected to a 5 percent silicon, it is clear that the ratio of the notch tensile percent salt spray test and exhibited only a few scattered rust strength to the smooth tensile decreases to a value below 1. spots after 1,000 hours exposure. Transverse sheet specimens From the foregoing it is clear that the steel of the present inwere evaluated after aging at temperatures of 900l ,000and vention possesses an outstanding combination of mechanical 10 l,050 F. at stresses equal to 70 percent of the ultimate tensile properties which make the steel desirable from a structural strength. In the stressed condition, these samples were substandpoint, so long as the chemical composition is limited as jected to the 5 percent salt spray test to evaluate the stress cortaught hereinbefore. As clearly demonstrated, the silicon and rosion resistance, and no failures were observed over a test manganese content must be limited in order to obtain this outperiod having a duration of 1,120 hours. Thus it is clear that standing combination of mechanical properties and a NS/UTS the steel of the present invention exhibits good corrosion reratio of at least I. sistance and excellent stress corrosion resistance.

From the standpoint of fabrication, the steel of the present Machinability of the steel of the present invention has been invention exhibits eXcellem Welding prop Heat evaluated against the machinability of commercially available having a composition which includes 0.03 percent carbon, steel, referred to hereinafter as steel C, and having a composi- 0.22 percent manganese, 0.20 percent silicon, 15.25 percent tion of 0.064 percent carbon, 0.36 percent manganese, 0.70 chromium, 6.02 percent nickel, 0.7 percent titanium and the percent silicon, 15.73 percent chromium, 4.22 percent nickel, balance essentially iron with incidental impurities, was 3.49 percent copper and the balance iron with incidental improcessed into strip material having a thickness of 0.031 inch, purities. annealed at 1,500 F. for a period of about 10 minutes, and 2s The following test conditions were utilized in performing thereafter cooled to room temperature. The strip was butt drilling tests on the steel of the present invention, as well as on welded by the automatic tungsten inert gas process utilizing no steel C, identified hereinbefore: filler material. Liquid penetrant inspection of all welds revealed no cracking in the weld nor in the base metal. Drill Dimmer y in Duplicate sheet tensile test specimens were machined with the Drill Material Type M! weldment longitudinally through the center of the test Point Angle specimen and transversely across the center of the reduced 22:23a gl i 43A section. These materials were pulled in the standard tenslle End poim 0.015 in. wearland on cuning test, and the following test results were recorded: edge TABLE VII Re Hardness Percent Y s, UT elonga- Location of Heat treatment Type test Base metal Weld K p.s.l K p.s.l. tlon fracture Annealed 1,5 F Unwelded 2s 114.2 139. 7 3.0 As weld d Transversely we1ded 2s 28 115.8 146. 5 3.0 Base metal- 0. Longltudlnally welded. 28 28 111. 7 137.9 Aged 8 hrs. at 900 Unwelded 1. 42 192. 4 192. 6 2. 8 Welded DlllS aged Transversely welded. 42 192.6 193. 0 2. 5 D0.

D0 Longltudlnally welded. 42 45 190.1 195. 5 6- Welded plus annealed at 1,500 F. plus aged" Transversnly welded. 41 40 188. 6 189.2 8.0 Weld.

D0. Longltudlnally welded 41 40 173.2 176.0 2. Aged 2 hrs. at 1,000 F Unwelded 39 168.2 169.1 5. 7 Welded p us aged Transversely welded 39 4a 170. 5 172.1 4.5 Base meta DO. Longltudinally welded 39 43 171. 3 176.2 5

It is clear from the first three entries of Table VII that very little change is effected to the tensile properties resulting from welding over those illustrated by the annealed tensile properties of the material in the unwelded condition. When the unwelded, annealed sample is thereafter aged for 8 hours at 900 F. the 0.2 percent yield strength increased from about 114.2 Kpsi to about l92.4 Kpsi. The samples which were annealed, welded and thereafter aged for 8 hours at 900 F. show substantially equivalent properties as the unwelded material. This is true whether or not subsequent to welding the material is reannealed and thereafter aged. It is clear from the test results recorded in Table VII that no post-weld annealing heat treatment is necessary. However, in order to obtain optimum strength, it is preferred to age the welded materials within the same low aging temperature range.

The steel of the present invention was subjected to some of the standard corrosion resistance test, and table VIII compares and contrasts the testing media with the steel having a composition within the limits of the present invention with that of AISI Type 430 stainless steel:

Utilizing the above conditions, drilling tests were made, the results of which are set forth graphically in the accompanying diagram.

It is apparent that the steel of the present invention has excellent machinability properties from the standpoint of tool life wherein a constant total depth of 10 inches is utilized. These test results further show that the steel of the present invention has outstanding machinability properties, even in the age hardened condition. These machinability properties far exceed those of steel C, both in the annealed and in the aged condition. Where constant drilling speed is utilized, the same results are apparent. Curve 10 shows the effect of drill speed on the average inches drilled for steel C after annealing at l,900 F. for l hour followed by an oil quench. When steel C is further heat treated, that is, after aging at 875 F. for 1 hour and air cooling and the same drilling tests are performed, the machinability of steel C is adversely affected, as clearly illustrated by curve 12 which has been displaced downwardly and to the left from curve 10. When the steel of the present invention is subjected to the same drilling test in the hot rolled, annealed and age hardened conditions, it exhibits far superior machinability characteristics. Curve 14 illustrates the machinability of the steel of the present invention in the hot rolled condition. It is clear that in this condition the steel of the present invention has far superior machinability characteristics than that of steel C. Curve 16 illustrates the machinability of the steel of the present invention in the annealed condition. Thus it is clear from a comparison of curve 10 with curve 16 that the steel of the present invention has excellent machinability. In contrast to steel C, which has poor machinability in the age hardened condition, the steel of the present invention has improved machinability over that of the annealed condition, as clearly illustrated by curve 18. The test results thus graphically set forth in the drawing, and as confirmed by the lathe turning test, clearly demonstrate the outstanding machinability characteristics of the steel of the present invention.

As set forth hereinbefore, the steel of the present invention is made without employing any special skills or equipment in the manufacture thereof. The heat treatments are simple and are advantageous from the standpoint that little distortion and little scaling or discoloration occur through producing the steel in its condition wherein it exhibits the optimum combination of mechanical properties.

The steel of the present invention has been produced in various forms including bars, sheet, strip, wire and seamless tubing, employing the normal commercial steel mill practices.

We claim:

1. A martensitic, age hardenable stainless steel having a balanced composition substantially free of delta ferrite; said steel consisting essentially of, traces to about 0.07 percent carbon, traces to about 0.35 percent silicon, traces to about 0.6 percent manganese, from about 14 percent to about 17.5 percent chromium, from about 4.75 percent to about 7 percent nickel, from about 0.3 percent to about 1.3 percent titanium, and the balance essentially iron with incidental impurities, the steel being characterized by exhibiting a ratio of notch tensile strength to smooth tensile strength of at least 1, a capability of developing a 0.2 percent yield strength substantially in excess of 100,000 psi upon aging, and good weldability.

2. A martensitic age hardenable stainless steel having a balanced composition substantially free of delta ferrite; said steel consisting essentially of, traces to about 0.05 percent carbon, traces to about 0.20 percent silicon, traces to about 0.3 percent manganese, from about 14.5 percent to about 15.5 percent chromium, from about 5.25 percent to about 6.5 percent nickel, from about 0.4 percent to about 1.0 percent titanium, and the balance essentially iron with incidental impurities, the steel being characterized by exhibiting a ratio of notch tensile strength to smooth tensile strength of at least 1, a capability of developing a 0.2 percent yield strength substantially in excess of 100,000 psi upon aging, and good weldability.

3. A martensitic, age hardenable stainless steel having a balanced composition substantially free of delta ferrite; said steel consisting essentially of, traces to about 0.07 percent carbon, from traces to about 0.35 percent silicon, from traces to about 0.60 percent manganese, from about 14 percent to about 17.5 percent chromium, from about 4.75 percent to about 7 percent nickel, from about 0.3 percent to about 1.3 percent columbium, and the balance essentially iron with incidental impurities, the steel being characterized by exhibiting a ratio of notch tensile strength to smooth tensile strength of at least 1, a capability of developing a 0.2 percent yield strength substantially in excess of 100,000 psi upon aging, and good weldability. 

2. A martensitic age hardenable stainless steel having a balanced composition substantially free of delta ferrite; said steel consisting essentially of, traces to about 0.05 percent carbon, traces to about 0.20 percent silicon, traces to about 0.3 percent manganese, from about 14.5 percent to about 15.5 percent chromium, from about 5.25 percent to about 6.5 percent nickel, from about 0.4 percent to about 1.0 percent titanium, and the balance essentially iron with incidental impurities, the steel being characterized by exhibiting a ratio of notch tensile strength to smooth tensile strength of at least 1, a capability of developing a 0.2 percent yield strength substantially in excess of 100,000 psi upon aging, and good weldability.
 3. A martensitic, age hardenable stainless steel having a balanced composition substantially free of delta ferrite; said steel consisting essentially of, traces to about 0.07 percent carbon, from traces to about 0.35 percent silicon, from traces to about 0.60 percent manganese, from about 14 percent to about 17.5 percent chromium, from about 4.75 percent to about 7 percent nickel, from about 0.3 percent to about 1.3 percent columbium, and the balance essentially iron with incidental impurities, the steel being characterized by exhibiting a ratio of notch tensile strength to smooth tensile strength of at least 1, a capability of developing a 0.2 percent yield strength substantially in excess of 100,000 psi upon aging, and good weldability. 